1. Field of the Invention
The invention relates to a test apparatus and a method for nondestructive material testing using electrodynamically generated ultrasonic waves.
Ultrasonic testing is a method of nondestructive material testing using ultrasound to detect cracks, inclusions, inhomogeneities, and other flaws. The ultrasound is generated piezoelectrically or electrodynamically, for example.
2. Description of the Related Art
In electrodynamic ultrasound generation, the ultrasound is generated directly in the test specimen, so that a coupling medium is not needed. The creation of the ultrasonic oscillations can be ascribed to the interaction of high-frequency eddy currents with a magnetic field. The eddy currents are generated for instance by a high-frequency coil assembly that is placed in the vicinity of the surface of the workpiece. Lorentz forces are created by means of a simultaneously operative magnetic field which produce sound waves in the workpiece. Depending on the orientation of the magnetic field and the eddy-current-generating coil relative to one another, longitudinal waves and arbitrarily polarized transverse waves can be induced. In the longitudinal wave, the direction of propagation and vibration are identical, while conversely in the case of the transverse wave the direction of the vibration is perpendicular to the direction of propagation. The transverse wave is also known as a thrust or shear wave which propagates only in solid media.
If the direction of polarization is located in the plane defined by the surface normal of the workpiece and the direction of propagation of the ultrasound, then one speaks of vertically polarized transverse waves. Conversely, if the direction of polarization is perpendicular to that plane, one speaks of horizontally polarized transverse waves. For use in the testing industry, horizontally polarized transverse waves can be generated only by electrodynamic excitation.
With electrodynamic ultrasound generation, it is possible to test the workpiece at temperatures up to about 1000K.
German Patent DE 42 04 643 discloses a test apparatus with a perpendicularly oriented permanent magnet, whose orientation changes in checkerboard fashion. The direction between the north and south poles of the permanent magnet is defined as the orientation. In this apparatus, the high-frequency coil is disposed in meandering fashion between a surface of the workpiece and the permanent magnet. This test apparatus is very complicated and expensive to manufacture, since the transmitting or receiving coil in flat form must be wound very thinly. At the previously conventional testing frequencies of approximately 0.7 MHz, this can be achieved only at very major effort and expense. For testing thin-walled components and pipelines, however, frequencies between 1 and 2 MHz are usual. To achieve this, the permanent-magnet and high-frequency coil arrangements must be reduced in size in accordance with the frequency. This makes the replicable production of such test apparatuses very complicated and expensive.
European Patent Disclosure EP 0 579 255 discloses a further test apparatus, in which the eddy currents required to excite sound are induced via a magnet yoke that encloses the permanent magnets. The distance between the two pole pieces of the magnet yoke is undesirably great. Thus that test apparatus is highly dependent in its efficiency, with respect to sound excitation and sound reception, on the material to be tested. For instance, satisfactory results have thus far been unattainable with nonmagnetic components. Yet precisely in the case of nonmagnetic weld seams and mixed seams, the use of horizontally polarized waves, which can be generated practically only electrodynamically, is especially suitable because of the columnar crystals through which the ultrasonic transmission is to take place.